This is a guess. In baking powder, the soda and acid is balanced for the desired effect, which is correct generation of carbon dioxide at the proper time. The nature of the acid also determines when that time is, and double action baking powder has one that becomes active at room temperature and a second that activates with heat. Anything that upsets the expected balance of acid alters the actions. Milk is normally slightly acidic. Tap water can be anything, but is generally more of less from slightly acid to slightly alkaline but can begin as more either way, depending on the makeup of bedrock in the regiom. But cities sometimes raise the pH as high as 9 to reduce pipe damage. If your water is very alkaline, it could neutralize the acid in baking soda, and there would be no rise. Or I suppose if the water was very acid, it could exhaust the soda too early. But I more likely suspect alkaline tap water, "hard" water being very common. Knowing the pH of your tap water is a good thing for folks baking a lot with baking powder. Mine is very hard. I paved my drive with what I chipped out of the bottom of my tea pot.
(Pure water is slightly acid by chemical nature, but it's only coincidence is any given water you find matches that. Distilled water is usually significantly acid, because it immediately begins forming carbolic acid.)